1. Field of the Invention
The present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In the basic structure of such a light-emitting element, a layer which contains a substance having a light-emitting property is interposed between a pair of electrodes. By voltage application to this element, the substance having a light-emitting property can emit light.
Since such light-emitting elements are self-luminous elements, they have advantages over liquid crystal displays in having high visibility of pixels and eliminating the need for a backlight, for example, thereby being considered as suitable for flat panel display elements. Also, above-described light-emitting elements have an advantage in that they can be thin and lightweight, and also have a feature of very high speed response.
Furthermore, since such light-emitting elements can be formed in a film form, they make it easy to provide planar light emission, thereby achieving large-area elements utilizing planar light emission. Such a feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements can be very effectively used as planar light sources applicable to lightings and the like.
Light-emitting elements using electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a substance having a light-emitting property.
In the case where the substance having a light-emitting property is an organic compound, voltage application to a light-emitting element enables injection of holes from an anode and electrons from a cathode into a layer containing the organic compound having a light-emitting property, so that a current flows. Then, the carriers (electrons and holes) recombine, whereby the organic compound having a light-emitting property is brought into an excited state. When the excited state returns to a ground state, light is emitted. In general, an organic EL element refers to such a light-emitting element which uses an organic compound having a light-emitting property and can be excited with a current.
Excited states of organic compounds can be a singlet state and a triplet state. The ground state of organic compounds that are generally used for organic EL elements is a singlet state. Light emission from a singlet excited state is called fluorescence, while that from a triplet excited state is called phosphorescence.
Proposal of a heterostructure in which layers of different organic compounds are stacked has brought about significant development of such light-emitting elements (see Non-Patent Document 1). That is because adopting a heterostructure increases carrier recombination efficiency and then improves emission efficiency. In Non-Patent Document 1, a hole-transport layer and a light-emitting layer having an electron-transport property are stacked.
Further, considerable researches have been conducted on correlation of the heterostructure with drive voltage or with lifetime. For example, it was reported that, in an element having a hole-transport layer in contact with an anode, the ionization potential of the hole-transport layer affects the lifetime (see Non-Patent Document 2). The element disclosed in Non-Patent Document 2 can have a longer lifetime as the ionization potential of the hole-transport layer decreases. It was also reported that the lifetime of an element is extended by providing a hole-injection layer having a low ionization potential between an anode and a hole-transport layer (see Non-Patent Documents 3 and 4).